342 PROCEEDINGS OF THE AMERICAN ACADEMY. 



connected in some way with the appearance of the new variety of ice, 

 but the exact connection cannot at present be stated. 



The large change in the vahie of the compressibiUty brought about 

 by pressure should be noticed, amounting at 12,000 kgm. to a decrease 

 of five fold. Furthermore the rapid flattening of the curve at the 

 higher pressures also should be commented on. The curve gives the 

 appearance, for the pressure ranges used here, of becoming asymp- 

 totic to some value greater than zero. Of course this cannot really 

 be the case for infinite pressures, for otherwise we should have the 

 volume completely disappearing for some finite value of the pressure, 

 but it may indicate the entrance of another effect at the higher pres- 

 sures, which may persist in comparative constancy for a greater range 

 of pressure than will ever be open to direct experiment, such an effect 

 as the compressibility of the atom, for example. This possibility 

 has been already mentioned and made plausible from the data of the 

 preceding paper. 



If instead of the compressibility as defined above, the quantity 



- ( — ) , which in this paper will be called the relative compressibility, 

 V \dpjt 



is plotted, a curve of the same general character as that shown will 

 be obtained. 



The compressibility may also be plotted against a different argument 

 than the pressure. For many purposes the pressure is perhaps not 

 the most significant independent variable that might be chosen. 

 This is because the external pressure is not a measure of what is 

 happening inside of the liquid. We conceive a liquid as composed of 

 molecules in a state of constant motion and of collision with each other, 

 acted on also by attractive forces between each other. The effect of 

 these attractive forces is to produce at the interior points a pressure 

 which may be much higher than the external pressure. The external 

 pressure is equal to the interior pressure diminished by the amount 

 of the attractive pressure drawing the molecules to the interior at the 

 exterior surface, where the attraction is an vmbalanced action in one 

 direction. The amount of the unbalanced pressure at the outside 

 depends in a complicated way on the law of attraction between the 

 molecules, on their mean distance apart in this surface layer, and on 

 the distribution of velocities in this layer. The external pressure 

 required to hold the liquid in equilibrium is, therefore, largely a sur- 

 face phenomonon, and is connected in a complicated way with the 

 state of affairs at inside points. A more significant independent 

 variable, therefore, would be one involving only the condition of the 



